Multi-layer oriented films for packaging applications having a polyester outer layer are widely known in the patent literature. These films generally comprise, in addition to the polyester first outer layer, a second outer layer of a heat-sealable material, such as a polyolefin layer, and preferably also a core gas barrier layer, such as a PVDC, EVOH and/or polyamide layer.
Examples of multi-layer oriented films that have a polyester first outer layer, a core gas-barrier layer of e.g. EVOH and/or polyamide, a polyolefin second outer layer and tie layers to adhere the various layers are described for instance in EP-A-476,836, WO 99/55528, WO 99/44824, WO 99/44823, EP-A-1,190,847, WO 01/98081, and WO 05/011978.
In particular, EP-A-476,836 describes an oriented laminated film with a surface layer of a polyester, an EVOH core layer, an intermediate layer of certain polyamides, with a given thickness ratio between the outer polyester layer and the core polyamide layer, and a heat-sealing layer of polyolefin. Tie layers of an acid modified ethylene-α-olefin copolymer (M-PE) are then used to bind the outer polyester layer to the intermediate polyamide layer.
WO 99/44823 describes EVOH-containing heat-shrinkable films with at least four layers, i.e., a first outer layer possibly comprising a polyester, a second outer layer which may comprise a polyethylene, a core EVOH layer and an additional core polyamide or polyester layer wherein an adhesive layer, such as a layer of modified ethylene-methyl acrylate is used to bind the outer polyester layer to a core layer.
WO 99/55528, EP-A-1,190,847, and WO 01/98081 relate to heat-shrinkable structures where in addition to the polyester and polyolefin outer layers, a core polyamide layer is present and optionally also an EVOH layer and a tie layer of e.g., a modified ethylene-α-olefin copolymer, is used to bind the outer polyester to a core layer.
WO 05/011978 describes high modulus bi-axially oriented films comprising a first outer layer which comprises a polyester, a second outer layer comprising an ethylene or propylene homo- or co-polymer, a core layer comprising EVOH, and no core polyamide or polyester layers. Also in this case the outer polyester layer is generally bound to a core layer by an adhesive layer.
In all these films the use in the outer layer of one or more ring-containing polyesters or co-polyesters characterized by a % of crystallinity relatively high, at least >15, and preferably >20, gives a number of different advantages to the end structures, such as a remarkable abuse resistance, easy printability, very good optical properties, in particular gloss, etc.
Depending on the particular resins employed and of the particular end applications desired, these films can be manufactured by coextrusion, extrusion coating, and/or lamination of preformed cast films, followed by mono- or biaxial orientation, and optionally by an annealing or heat-setting step, or they may be prepared by lamination of preformed films of which at least part have been mono- or bi-axially oriented. In most of these process, but in particular when the films are obtained by coextrusion followed by orientation, adhesive or tie layers, typically polyolefin-based, in particular polyethylene- or propylene-based, are employed to increase the adhesion (also called “bond”) between the various layers.
It has been found however that, in case of oriented structures, either heat-shrinkable or heat-set, the bond strength between a polyester or co-polyester layer of relatively high % of crystallinity and such a conventional adhesive layer is sometimes not sufficiently high to avoid possible delamination problems.
It has been found—and this is the gist of the present invention—that it is however possible to increase such a bond by positioning between the layer of polyester or co-polyester of relatively high % of crystallinity and the adjacent polyolefin-based adhesive layer, a layer of a co-polyester characterized by a relatively low % of crystallinity.